Power MOSFETs in inductive circuits will frequently go into avalanche during turn off if Eoff (turn-off energy) is to be limited. Avalanche current has a high di/dt and produces high frequency oscillation in the range of 10 MHz to 30 MHz and is a source of EMI radiation.
More specifically, DC-Loop parasitic inductance (Lp) stores energy when a FET in the loop conducts. This energy is discharged in the FET when it turns off. Lp begins to discharge when the drain source voltage Vds exceeds the bus voltage. When Vds reaches BV (breakdown voltage) the FET avalanches. Energy stored in Lp is the lower limit of Eoff In order to get the lowest Eoff that can be achieved with a hard switching circuit, the FET should avalanche at turn-off. Avalanche, however, produces high di/dt and is followed by high frequency voltage oscillations in the range of 10 MHz to 30 MHz. Therefore, to minimize these potential EMI sources avalanche should be avoided.